In a distillation unit in a petroleum refining operation, hydrocarbon-containing feedstocks are ordinarily separated in distillation columns (towers) by at least a portion of a liquid being vaporized and the vapor then being physically separated from the liquid and usually condensed to a liquid, either within or outside the column. For instance, a full-range naphtha containing a heavy and light boiling fraction is heated in a distillation column to vaporize the light naphtha fraction which is recovered overhead while the heavy naphtha liquid fraction is recovered from the bottom. The distillation columns typically contain either trays or packings to regulate fluid flow through the column. The usual purpose of packings in a packed refinery distillation column is to bring a current of gas or vapor into intimate contact with a liquid stream and vice versa, and create flow paths for the liquids. In general, packed refinery distillation columns are operated countercurrently, that is, the liquid enters the tower at the top, where it is distributed over the packing by means of a specially designed distributor, and the gas or other liquid enters at the base of the column.
In a combined catalyst-distillation process as disclosed in U.S. Pat. No. 4,215,011 issued to Smith, a catalyst bed contained in a distillation column is contacted with a feedstock, such as a mixture of isobutene and n-butene, under reaction conditions to chemically change a substantial proportion (more than a trace) of the feedstock components (i.e., greater than 0.01 weight percent and typically a major proportion of the feedstock components) to convert them to at least one different product from that contained in the feedstock, such as diisobutene, and the product is concurrently removed from the column by distillation. Such a process and other processes of the same nature typically involve equilibrium-driven reactions wherein the forward reaction has an increased driving force because the reaction products have been removed and cannot contribute to a reverse reaction (LeChatelier's Principle).
However, in the common scheme of refining crude oil, both crude and processed hydrocarbon-containing boiling fractions are physically separated by distillation in packed columns into product hydrocarbons, without change of chemical composition of the feedstock compounds, and subsequently are catalytically upgraded to more valuable hydrocarbon-containing products. For instance, a vacuum gas oil fraction is catalytically hydrocracked and the products subsequently distilled to boiling fractions such as diesel fuel, jet fuel, heavy naphtha and light naphtha.
Such cracked naphthas, as well as straight run naphtha feedstocks (including light, heavy and full range naphthas), are often upgraded in other processes, such as catalytic reforming and isomerization, by increasing the octane number of the feedstock's gasoline fraction. In a typical reforming process in which a straight run or cracked naphtha is upgraded, the feedstock is contacted with a catalyst comprising a noble metal on alumina. The conditions utilized in the reforming process will vary depending upon, among other factors, the type of feed being processed and the desired increase in octane level. The catalysts employed in the reforming process, particularly those containing platinum, and most particularly those comprising platinum, rhenium and chlorine, are poisoned or deactivated rapidly in the presence of sulfur components. Thus, to achieve maximum run lengths and increase process efficiency, it is necessary to reduce the sulfur content of reformer feedstocks as low as possible.
One common method of removing sulfur compounds from reformer and related feedstocks is to subject the feedstock to catalytic hydrodesulfurization by contacting the feedstock with molecular hydrogen in the presence of a sulfur-tolerant hydrotreating catalyst. The sulfur compounds in the hydrocarbon stream are converted to hydrogen sulfide, which may be separated from the hydrocarbon stream by conventional means prior to subjecting it to reforming. Although highly effective sulfur removal may be achieved by catalytic hydro-desulfurization, the efficiency of the process is ultimately limited by equilibrium and/or kinetic considerations. In general, it is not possible to obtain hydrodesulfurized products containing less than about 0.5 ppmw sulfur as is desired in most reforming operations. Furthermore, it is impossible to guard against upsets in the catalytic hydrodesulfurization units which can result in high levels of organosulfur compounds (such as mercaptan sulfur) remaining in the reformer feedstock.
In addition to being highly sensitive to sulfur components, reforming catalysts are also poisoned by compounds containing silicon. One common source of hydrocarbon streams containing silicon compounds is the delayed coking unit used to convert residual oils into more valuable products. The overhead vapors from the coking drum, which is part of the delayed coking unit, are normally distilled (fractionated) into various cuts including a gasoline boiling range stream commonly referred to as coker gasoline or coker naphtha. This stream generally possesses a low octane number and is therefore unsuitable for use as an automotive fuel without upgrading. Thus, it has become common practice to increase the octane number of coker gasoline by subjecting it to catalytic reforming. The coker gasoline will not only normally contain sulfur compounds but, quite frequently, will contain organosilicon components derived from silicon defoamers, such as polydimethyl siloxanes, added in the delayed coking process to prevent foaming.
In view of the above, it is desirable to remove contaminants such as sulfur compounds and silicon compounds from coker gasoline, jet fuels, and other hydrocarbon streams, including straight run and cracked naphthas (i.e., products from crude oil fractionation, fluid cracking catalysts, hydrocracking, and the like) that are to be processed in catalytic reformers or isomerization units. If a feedstock stream containing both sulfur and silicon compounds is subjected to catalytic hydrodesulfurization or hydrotreating, the sulfur will not only be removed by conversion to hydrogen sulfide but the silicon compounds will deposit on the catalyst. It is, however, not desirable to use the hydrotreating catalyst to remove silicon components from reformer feedstreams. After a certain amount of silicon deposits on the catalyst, complete removal of silicon compounds will no longer take place, and the effluent from the hydrotreating unit will contain at least trace concentrations of organosilicon components which will irreversibly poison the reforming catalyst. Moreover, the deposited organosilicon components will have a deleterious effect on the hydrotreating catalyst, tending to poison it and decrease its effectiveness.
In addition to hydrotreating, there are other processes, particularly hydrodesulfurizaton, employing catalytically active materials for removing sulfur and/or silicon from hydrocarbons. Such processes reduce the sulfur or silicon content of the hydrocarbon by "absorbing" sulfur therefrom by employing a sorbent material under nonhydrogenative conditions in a separate packed bed absorption unit located upstream of the reforming unit. Typically, nonhydrogenative conditions include contact of the sorbent material with the feedstock in the absence of hydrogen; however, if desired, hydrogen is sometimes present, but only in amounts or under conditions that prevent essentially any hydrogenation of the organosulfur components in the feedstock. Usually, the sorbent material contains a metal component, such a nickel, copper, zinc or silver, and the feedstocks generally treated are reformer feedstocks, particularly naphthas. Typical of such processes include those disclosed in U.S. Pat. Nos. 2,755,226 and 4,695,366 to Annable and Miller et al., respectively, wherein a bed of copper components supported on porous carriers is used to reduce the sulfur content of reformer feedstreams.